1. Field of the Invention
The present invention relates to an improved process for preparing 3,7-dimethyl-2,6-octadien-1-al (citral) by thermal cleavage of 3-methyl-2-buten-1-al diprenyl acetal.
2. Description of the Background
Disregarding the improvements according to the invention, this reaction has already been disclosed in its essential features, Inter alia, by DE 24 11 530 and EP 210 074 B1.
This is a highly complex reaction which proceeds via three stages, as follows from the reaction diagram below.
1. Thermal cleavage of the acetal of the formula II into 3-methyl-2-buten-1-ol (prenol) of the formula III and the cis/trans-prenyl 3-methylbutadienyl ether (dienyl ether) of the formula IV ##STR5## PA0 2. Thermal rearrangement of the dienyl ether of the formula IV in a Claisen rearrangement to give 2,4,4-trimethyl-3-formyl-1,5-hexadiene (formylhexadiene) of the formula V ##STR6## PA0 3. Thermal rearrangement of the dienyl ether of the formula V in a Cope rearrangement to give the desired citral of the formula I ##STR7##
Citral is used as a scent and, furthermore, is of great importance as a starting material for other scents, such as geraniol, and as a starting material for vitamins, for example vitamin A.
Since not only the starting compound of the formula II but also the desired citral, the byproduct prenol and the citral precursors of the formulae IV and V are sensitive substances which can react further in an undesirable manner, the preparation of citral initially succeeded only with yields of from 60 to 70%. Better citral yields by far were obtained, even on an industrial scale, in accordance with the process of EP 0 021 074 B1. This describes a process for preparing citral from the acetal of the formula II, which comprises continuously distilling off from the reaction mixture, during the reaction, the prenol formed as byproduct. The citral and the citral precursors of the formulae IV and V are retained in the reaction mixture in this case by a distillation tower. The continuous removal of the prenol from the reaction mixture increased the citral yield to about from 85 to 90%. To increase the citral yield further, in EP 0 021 074, in addition, the conjoint use of a substance having a boiling point between that of prenol and that of citral and the citral precursors of the formulae IV and V (what is termed the intermediate boiler) was recommended. This measure increased the citral yield to approximately 95% of theory.
A disadvantage of the process described, which is very good per se, is that the process is only suitable for a batchwise procedure and requires long residence times (about 6 hours) and therefore large apparatuses. A further disadvantage of the known process is that an auxiliary, that is to say the intermediate boiler, must be introduced into the system, which means additional resources for metering the auxiliary, for monitoring its concentration and removing it.